n = int(input())
a = list(map(int, input().split()))
count = 0
for i in range(1, n-1):
  if a[i] > a[i-1] and a[i] > a[i+1] and not a[i-1] == a[i+1]:
    count += 1
  elif a[i] < a[i-1] and a[i] < a[i+1] and not a[i-1] == a[i+1]:
    count += 1

print(count)